Implement optional type parameters (#360)

* Add a NATIVE<T> macro type to simplify use of a native WebAssembly type
* Add default type parameters for internal helpers for explicit loads and stores
* Unify loadUnsafe/loadUnsafeWithOffset etc. into one
* Renamed loadUnsafe etc. into just LOAD, like a macro
* Implement parsing of index signatures, but ignore them, for properly linting code
* Refactor TypedArray<T> to use macros

tests/parser/optional-typeparameters.ts.fixture.ts

@@ -0,0 +1,10 @@
+function a<T=i32>(): T {
+  return 0;
+}
+function a<T, U=i32>(): T {
+  return 0;
+}
+function a<T, U=i32, V>(): T {
+  return 0;
+}
+// ERROR 2706: "Required type parameters may not follow optional type parameters." in optional-typeparameters.ts:3:19